1. Field of the Invention
The present invention relates to the field of rocket-launched projectiles. More particularly, the invention is directed to a system and method for reducing the dispersion of rockets by using lateral thrusters to oppose any initial yawing motion. This system is self-contained and can be retrofitted to existing unguided rocket systems.
2. Description of Related Art
It is well known in the art that rocket-launched projectiles are subject to delivery inaccuracies. Factors contributing to projectile delivery inaccuracy are usually divided into two categories: mean point of impact (MPI) errors and precision errors. MPI is the average of the impact locations of a group of projectiles fired from a single launcher on a particular occasion. The MPI error captures the variability, over multiple occasions, of the MPI about the aim point. MPI error is also called occasion-to-occasion or mission-to-mission error.
Precision error is the variation of individual round impacts within a volley about the MPI. Precision error is primarily attributable to variances of projectiles and propellants resulting from allowable manufacturing tolerances. Precision error is also called random error or round-to-round dispersion.
Initial yawing rate is the largest contributor to the precision errors of small rockets. These round-to-round initial yawing motions are caused by such things as balloting-inducing tip-off and variable gravity droop. This angular motion produces yaw during the launch/thrust phase of small rockets"" flights. When a rocket is yawed, its thrust vector is not aligned with its velocity vector, causing the rocket to depart from its intended flight path in the direction it is pointing. Integrated over time, this produces a large dispersion error.
Yawing motion during rocket launch and thrust can also result from occasion errors such as meteorological (MET) variations and manufacturing lot variations. Platform characteristics contribute to occasion errors whenever small rockets employ a number of possible launchers mounted on different launch platforms.
For example, rigid ground-based launchers have very different occasion errors than do helicopter-borne launchers. Helicopter-launched rocket trajectories are affected by both the helicopter""s motion and by the rotor wake present at the time of launch. The helicopter""s motion at the time of rocket launch affects the rocket""s trajectory through the addition of the aircraft""s motion to the rocket""s initial motion. Further, depending on the type of helicopter and maneuver, rotor motion will vary. Instantaneous rotor wakes, i.e., downwash, vary with the rotor""s speed and rotational position. The magnitude and direction of downwash at launcher exit will affect the rocket""s initial yawing motion.
It is known that yawing motion of rockets can be quickly damped by a technique known as active damping, comprising appropriately timed firings of impulsive lateral thrusters in a direction that opposes the yawing motion. Kershner et al., U.S. Pat. No. 2,995,319, the entire disclosure of which is hereby incorporated by reference, teach a device for controlling roll, pitch, and yaw attitude. This device utilizes four jet reaction nozzles mounted at the rear of a missile, mechanically linked to the rocket""s tail fins. Bagley, U.S. Pat. No. 4,967,982, the entire disclosure of which is hereby incorporated by reference, teaches a lateral thruster design to be used for steering and maneuvering missiles during flight. Bagley""s device provides continuous lateral forces which are controllable in both magnitude and direction of application. However, both these patents teach providing control forces primarily intended for use in later portions of rocket trajectories rather than during the initial launch stage.
While Kershner and Bagley disclose devices for maintaining a rocket""s flight path, neither teaches improving rocket performance/accuracy. Further, neither of these patents is directed to a self-contained device in a configuration that can be retrofitted to existing missile systems. And, neither Kershner nor Bagley discloses an approach that dynamically measures and opposes excessive initial yawing rates in order to reduce the dispersion of rocket-launched projectiles.
The present invention builds on the technique of active damping to provide a system and method for damping yaw during initial rocket launch. To accomplish active damping during initial flight, the present invention employs a system comprising multiple thrusters, angular rate sensors, and a simple thruster firing circuit that are all contained in a single section that can be installed in a cylindrical section of the rocket body by insertion between other flight body parts.
The method of active damping of the present invention is intended to be completed before the occurrence of first maximum yaw of a rocket""s flight thereby reducing rocket dispersion. This method includes monitoring the missile""s initial yawing rate at launch, and, if that rate exceeds preset thresholds, firing a lateral thruster in a direction opposed to that yawing motion. This method includes repeating these monitoring and firing steps, as required, for a preset period immediately following launch.
The method of the present invention has been evaluated via simulation of a ground-launched rocket at 25xc2x0 elevation, without any initial disturbances and flying through a standard atmosphere. In this simulation, when the missile is 2000 m down range it will be at a height of 822 m and will have deflected 0.6 m to the right of the line of fire. For each of 950 simulated flights of rocket launches at 25xc2x0 elevation, initial yawing rates were randomly drawn from distributions representative of initial rates measured for such rockets. The height and deflection components of the positions of each of these rockets are shown in FIG. 3, where each rocket""s position is represented by a dot. The cross hairs indicate the nominal rocket position or aim point. The left axis gives the absolute height and the right axis the height relative to the aim point. Because the deflection component of the aim point is only 0.6 m, a relative deflection location axis was not included as it would be indistinguishable from the absolute deflection location axis. The extent of the rockets"" dispersion pattern can be read directly on the bottom and right axes. If these results are thought of in terms of rockets launched at a 2000 m distant aerial target, such as a hovering helicopter, at 822 m height, the graphed locations give the points of closest approach of the rockets to that target. The sample mean is 28 m low and 0.5 m to the right of the aim point and the sample deviations are 34.6 m in height and 30.8 m in deflection.
Another measure of dispersion commonly used within the military is the circular error probable (CEP). CEP is the radius of a circle, usually about the sample mean of the impacts, but also sometimes about the point of aim, which encloses 50 percent of the shots in the sample. The CEP about the sample mean lends itself to a simple graphical representation of both biases and relative dispersions of active damping concepts. The CEP for the shot pattern of the simulated undamped rockets is 32.4 m and is indicated by the circle in FIG. 3.
When the active damping system of the present invention was modeled the shot pattern shown in FIG. 4 resulted. This modeled active damping system has a sample mean 1.8 m above and 0.2 m to the left of the aim point, the sample deviations are 9.1 m in height and 11.5 deflection, and the 50 percent CEP radius is 8.0 m. No sensor errors, thruster initiation variations, or variations in thrust profiles were modeled. For this idealized simulation scenario, active damping virtually eliminated the bias in the mean and reduced the spread of the projectiles by about a factor of four.